LocoPollo,
There are two varieties of blues. One is the common blue which is the same as the blue Andalusian are based on.
The other is self blue which is difficult to get in large fowl and is what is also called Lavender in the UK.
The common blue you will have no difficulty into producing some blue sussex in a few years. Just try to find a blue of another breed that is very similar to your sussex breed. HS

LocoPollo,
It is not going to be that simple. A speckle sussex has a complicated genetic makeup. You may be quicker using a light sussex. You need to find someone on here that is more knowledgeable then me to work out the details. The speckle sussex will probably give you a lot of red to clean up to make a pure blue. HS